Loud speaker



Jane M,- 13..

F. a... CREAGER Elf AL 3.

LOUD SPEAKER Filed Jan. 30, 1932 INVENTORS. FredemcKLCr-emger;

J mm 4 THATTORNEY.

M. Lee McMurtrey,

Patented Jan. 14, 1936 UNITED STATES PATENT OFFICE LOUD SPEAKER ware Application January 30, 1932, Serial No. 589,824

4' Claims.

Our present invention relates to sound translating devices, and more particularly to loud speakers of the moving coil or dynamic type having a magnetic field produced either by a field coil, or by a permanent magnet pole structure.

In the construction of dynamic sound reproducers with which we are familiar, it has been the practice heretofore to secure together the various elements of the field yoke or pole structure and the cone diaphragm housing by means of a plurality of bolts and rivets, and to provide a centering ring between the pole pieces forming the air gap. For the purpose of spacing the outer pole piece or yoke plate from the inner pole piece or core in order to form an annular air gap in which the moving coil of the dynamic sound reproducer is adapted to vibrate, it has been customary to form a metal shading ring of low resistance non-magnetic material and attach the ring to the yoke plate, as by means of rivets, about and below the air gap opening in the plate. Thereafter the opening, of reduced diameter, in the ring has been bored in accurate concentricity with respect to the opening in the yoke plate. The plate and ring assembly has been retained in place on the yoke by means of a plurality of bolts, the shading ring serving to space the pole pieces as well as to function as a short-circuited turn on the core.

An assembly of the above type has been found to be relatively costly in manufacture because of the number of operations required, including forming the shading or centering ring, drilling the yoke plate for the various rivets or bolts to hold the ring in place, the subsequent boring of the ring after its assembly on the plate, and the assembling of the various parts of the speaker by means of bolts and the like.

In the construction of permanent magnet field units heretofore, using hard chrome-steel, difficulty has been experienced in'providing the necessary holes for securing the elements together. For example, it has been necessary to soften the metal by heat-treating in order to provide. the usual hole in the bottom of the yoke for securing the center pole piece thereto. It has been found that this operation weakens the material, and upon subsequent hardening of the metal there has been a tendency for the yoke to deform from its previous shape.

In another construction with which We are familiar and in which the spacing ring has been omitted, it has been customary to prevent displacement by the use of doweling means for securing the pole pieces in spaced relation to form the air gap for the moving coil, relying upon bolts as in the above mentioned cases, for securing together the elements of the pole structure.

It is, accordingly, an object of our invention to provide a magnetic field structure for an electrical sound producing device of the dynamic type whereby the manufacture and assembly thereof may be facilitated and the operations and adjustments incident thereto and requiring great skill may be obviated, thereby resulting in a device which may be manufactured in quantities with a high degree of uniformity.

A further and more specific object of our invention is to provide a magnetic field structure wherein it is possible to assemble the members thereof without the use of bolts and similar securing means, while permanently maintaining the critical relative positions of the members, thereby obtaining ruggedness of construction, and desired magnetic characteristics.

A still further object of our invention is to provide a magnetic field structure wherein the usual spacing ring of nonmagnetic material, ordinarily secured to the outer pole piece for spacing the inner pole piece therefrom, may be entirely eliminated.

A still further object of our invention is to provide an electromagnetic field structure wherein the customary short circuiting or shading ring, frequently used simultaneously as a spacing ring as above noted, may instead serve the more useful purpose of securing the field coil or electromagnetic winding in place on the inner pole piece.

Broadly considered, our invention comprises securing together the major elements of the magnetic field structure of a sound producing device, such as a dynamic speaker field yoke or pole structure, by welding, the elements being securely held during assembly in proper relative position and permanently maintained in such position preferably by fillet welding.

In a preferred construction, a shading ring is formed as a plain collar, retained on the center pole piece by a force or shrink fit. A magnetic field structure contemplated by the invention may include, as a shading ring, an internally threaded collar screwed onto a raised-thread portion of the center pole piece. In these constructions, the shading ring may not only be utilized to hold the field coil firmly in place, but may also function in its usual capacity as a Short circuited, low resistance turn to suppress the undesirable effects of any fluctuations in the magnetic field strength.

The invention will, however, be better understood when considered in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

Referring to the drawing, Figure 1 is a view in perspective, and on a reduced scale, of a sound producing device embodying our invention, and taken from an angle to the rearward;

Fig. 2 is a side view in section of the device shown in Fig. 1; and

Fig. 3 is a side view in section of a modification of the construction shown in Figs. 1 and 2,

' and embodying our invention.

Referring to Figs. 1 and 2, the sound producing device shown is an electrodynamic loud speaker, representing a preferred embodiment of our invention, and includes a U-shaped yoke member I of ferro-magnetic material in combination with a center pole piece 3, also of ferro-magnetic material, secured at one end to the yoke in any suitable manner, as by riveting, over the yoke as indicated in the drawing, a reduced diameter portion of the lower end of the center pole piece that extends through a hole 5 inthe yoke I. The end of the reduced diameter portion is upset by a special riveting operation that causes the inner pole piece to be securely held in place on the U-shaped yoke. It has been found that the riveting operation causes the metal of the reduced diameter portion within the bore of the yoke to expand or fiow and make a good low reluctance connection with the yoke member.

A yoke plate or outer pole piece I, having a central opening into which the center pole piece extends, is secured to the outer ends of the U- shaped yoke in a position wherein the opening is uniformly concentric with the end of the pole piece, thereby forming an annular air gap 9. For this purpose, during assembly, a suitable form (not shown) ground to accurate dimensions may be employed to accurately space the pole piece in the air gap. The end faces H of the ends of the U-shaped yoke are surface ground, and the yoke plate 7 is securely held in place on the ends of the yoke and a fillet type of arc weld, shown at I3, is applied as shown along the outer edge of the ends of the U, thereby securely holding the yoke plate to the yoke and forming a good magnetic path of low reluctance.

It might ordinarily be considered, on the part of those skilled in the art, that welding would cause formation of oxide coating or scale on the surfaces in contact and would also cause separation of surfaces because of warping. It has been found, however, that the magnetic path so formed is in no way impaired, but on the contrary is improved by the continuity of the bond. Although we prefer to employ fillet arc welding, any of the commercial types of welding may be employed by suitable changes in design details.

Prior to the assembly of the top yoke plate, a field coil l5, preferably having protecting insulating and cushioning pads such as felt rings IT and I8 at each end, is placed on the center pole piece, and a shading ring or plate l9, having a close-fitting central bore, is secured on the end of the central pole piece, preferably by a shrink fit, against the pad ll, thereby serving to hold the field coil in place on the yoke assembly.

A cone diaphragm 2| having a moving coil structure 23 and a centering spider 25 secured to the small end thereof is carried by a metallic frame or housing. The latter, for example, comforth.

prises a dished sheet metal strip 21 having its ends attached, as by welding, to one of a pair of annular rings 29 and 3| that are adapted to grip the periphery of a flexible supporting:.-eniber 33 attached to the large end of the cone in the usual manner. It has been found expedient to fasten the rings 29 and 3| together by rivets 35. The cone housing is secured to the pole structure preferably by spot-welding the dished support plate 21 to the yoke plate 1.

Referring to another form of speaker construction, in Fig. 3, the yoke plate 1 and supporting arms 21 of Figs. 1 and 2 for the rings 29 and ill are integrally formed from a single strip of ferro-magnetic material 28, thereby resulting in a further reduction in the number of parts and providing a rigid structure. The yoke plate portion of the strip 28 is fillet welded to the ends of the yoke at I3 as in the above case, and the ends of the arm portions of the strip 28 are adapted to be spot welded to the cone support ring, not shown. The center pole piece may also, if desired, be welded to the bottom of the yoke as shown at 39. It has been found convenient to secure the input transformer 41 to one of the arms of the strip 28.

The structure shown in Fig. 3, particularly the single piece construction of the yoke plate 28 and cone supporting arms, but having the inner pole piece fastened in place on the yoke as in Fig. 2, has been successfully employed in quantity production of a small sized, low priced electrodynamic speaker. In the case of permanent field dynamic speakers the feature of welding the center pole to the yoke has been found particularly desirable in that it obviates the necessity of forming a large hole in the yoke of hardened steel for securing purposes as above set In the welding of permanent magnet field yokes, however. the use of special materials and precautions has been found necessary because of the hardness of the steel. For the purpose of obtaining greater field strength, it has been found desirable in this type of speaker to increase the length of the yoke, as by expanding the U and welding the ends thereof to the ends of the arms of the strip 28.

From the foregoing description, it will be seen that, in accordance with the invention, a greatly simplified magnetic field structure may be provided for a sound producing device, and the construction is of a much more permanent and reliable nature than one constructed from parts adapted to be fitted together and secured by holding means such as bolts, rivets and the like:

It will be obvious that the above improved constructions result in a considerable reduction in the number of parts used, as well as a reduction in the assembling and manufacturing operations. The number of drilling operations has been reduced to a minimum and it has been found in every case that the cost of production has been reduced considerably.

While the invention has been illustrated and described in connection with its present preferred embodiment in an electrodynamic loud speaker, it should be understood that it is not limited thereto, but may be applied to any magnetic field structure for a sound producing device or the like having a moving coil or dynamic type.

We claim as our invention:

1. In a sound translating device of the dynamic type, a yoke of magnetic material comprising a central pole piece and an outer pole structure on either side of said central pole piece, and a yoke plate comprising a single fiat piece of metal having a central opening adapted to form with said central pole piece an annular air gap, said plate being united with said outer pole structure by fusion of magnetic material and having integral arms outwardly bent for supporting an acoustic diaphragm, said arms being substantially as wide as said yoke-plate.

2. In a magnetic field structure for a sound translating device of the moving coil type, a U-shaped yoke of magnetic material, a central pole piece of magnetic material, and a concave pole member of magnetic material united to said yoke and arranged to complete a magnetic circuit between said yoke and said pole piece, a fixed permanent operating air gap being formed thereby between said member and said pole piece, said concave member comprising a bent flat strip the width of which is substantially equal to the space between the legs of the U-shaped yoke plus twice the thickness of one leg and whose ends are adapted to support a diaphragm structure.

3. In a loudspeaker, a U-shaped yoke provided with a pole piece, a U-shaped member secured to said yoke by fusion and provided with a central opening receiving the end of said pole piece to form an annular air gap, at least a part of said member constituting a yoke plate for said loudspeaker, the width of said member being substantially the same throughout its length, and means secured to the extremities of said member for supporting an acoustic diaphragm.

4. In a sound translating device of the dynamic type, a U-shaped yoke of permeable material, a yoke-plate bridged across the ends of the yoke and affixed thereto by fusion, the said plate being formed of a single piece of flat permeable material and having a central opening, and a pole piece carried by the bottom of the yoke extending into said opening to provide an airgap, the extremities of the yoke-plate being 20 substantially as wide as the central portion thereof and being adapted to support an acoustic diaphragm.

FREDERICK L. CREAGER. M. LEE MCMURTREY. 

